Precharging of substrate for electrostatic printing

ABSTRACT

Various methods and apparatus are disclosed for the treatment of poorly conducting substrates with ions so that the substrate is more receptive to printing material and the printing material is more adherent to the substrate prior to being fused thereon.

Kenneth W. Rarey South Holland;

James G. Buck, Western Springs; John B.

Kennedy, Jr., Western Springs; Laszlo J.

Javorik, Chicago; Edward D. Higgins,

Palos Heights, 111.

[21] Appl. No. 618,395

[22] Filed Feb. 24, 1967 [45] Patented Feb. 9, 1971 [73] AssigneeContinental Can Company New York, N.Y.

a corporation of New York Continuation-impart of application Ser. No.396,060, Sept. 14, 1964, now Patent No. 3,306,193, which is aContinuation-'in-part of application Ser. No. 599,822, Dec. 7, 1966,which is a division of application Ser. No. 396,060 which is aContinuation-impart of application Ser. No. 409,213, Nov. 5, 1964, nowabandoned and a continuation-in-part of 609,275, Jan. 10, 1967, which isa continuation of application Ser. No.

[72] Inventors [54] PRECHARGING OF SUBSTRATE FOR ELECTROSTATIC PRINTING[51] Int. Cl 1341f 15/14 [50] Field ofSearch 101/114,

(ESD), 129, 426; 317/2; 313/325 [56] References Cited UNITED STATESPATENTS 2,940,864 6/1960 Watson 10l/ESD 3,241,483 3/1966 Duff l0l/ESD1,653,599 12/1927 Chapman 317/2 1,678,869 7/1928 Morrison..... 317/21,903,840 4/1933 Simons 317/2 2,087,915 7/1937 Kimball.... 317/22,163,294 6/1939 Simons 317/2 2,394,656 2/1946 Beregh 317/2X 2,944,1477/1960 Bolton lOI/ESD 2,965,481 12/1960 Gundlach IOI/ESD 3,081,6983/1963 Childress et a1. l01/ESD 3,102,045 8/1963 Metcalfe et al.-..101/ESD 3,273,496 9/1966 Melmon l01/ESD 3,295,440 l/1967 Rareyetal.101/114 3,296,963 1/1967 Rarey et a1. 101/114 3,306,193 2/1967 Rarey eta1. 101/114 3,306,198 2/1967 Rarey lOI/ESD Primary Examiner-Edgar S.Burr Attorney-Diller, Brown, Romik & Holt ABSTRACT: Various methods andapparatus are disclosed for the treatment of poorly conductingsubstrates with ions so that the substrate is more receptive to printingmaterial and the printing material is more adherent to the substrateprior to being fused thereon.

ATENTED FEB 9 I97! INVENTORS KENNETH w. EAQEY JOHN aueuuem, 1k,

JAMES G. BUCK,

LASZLO JJAVORW 8r" w FEM Q ERB'P flfi'ij ATTORNIZYfi PRECHARGING OFSUBSTRATE FOR ELECTROSTATIC PRINTING CROSS-REFERENCE TO RELATEDAPPLICATIONS 409,2l3, filed on Nov. 5, 1964, now abandoned; and acontinuation-in-part of application Ser. No. 609,275 filed Jan. 10, 1967as a continuation of application Ser. No. 409,213.

BACKGROUND OF THE INVENTION The invention relates to the field of coronaphenomena and particularly to the application of such phenomena to thefield of electrostatic printing.

Prior art electrostatic printing methods and apparatus utilize toner forproducing the desired image upon a substrate. The toner is generallyprovided with an electrostatic charge of a single polarity and, byelectrical means, is deposited upon a substrate. Consequently, initiallydeposited toner creates an electric field which tends to repeldeposition'of additional toner having the same polarity. Additionaltoner can be deposited if an electric field is utilized to effect thedeposition; however, during transfer of the printed substrate to afixing or fusing station, the substrate is removed from the influence ofthe electric field and part of the deposited toner is repelled from thesubstrate prior to the fixing or fusing operation. As a result,top-quality printing cannot be achieved by prior art devices.

SUMMARY OF THE INVENTION The present invention provides improved methodsand apparatus for depositing toner upon a substrate and for holding thetoner upon the substrate until it is fixed or fused thereon.

In addition, the invention provides improved apparatus for creating andmaintaining corona discharges and control means for governing thequantitative and qualitative distribution of ions upon an article to becharged.

Printing rates for poorly conducting materials such as paper, cardboard,paperboard and the like have heretofore been relatively slow, ascompared to the printing rates of the present invention, and have beenconsiderably slow under conditions of low relative humidity. With theambient atmosphere having a relative humidity of 20 percent, or less,printing rates greater than 100 impressions per minute could not beattained; the present invention provides for improved printing rates,and rates in. excess of I impressions per minute have been attained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic elevationalview illustrating one embodiment of the present invention;

FIG. 2 is a diagrammatic elevational view, partly in section, showingone operative embodiment of a printing machine embodying the presentinvention;

FIG. 3 is a diagrammatic elevational view illustrating a secondembodiment of the invention wherein the substrate is precharged at aprecharging location outside of the printing zone;

FIG. 4 is a diagrammatic elevational view illustrating anotherembodiment of the invention wherein the substrate is precharged outsideof the printing zone but on the side of the substrate upon whichprinting will occur;

FIG. 5 is a diagrammatic view of a second form of printing machineembodying the present invention;

' FIG. 6 is a diagrammatic perspective view illustrating one form of theinvention wherein a flow of air is used to move ions into contact with asubstrate to be charged;

FIG. 7 is a diagrammatic elevational view, similar to FIG. 6, butshowing the substrate as being charged on the surface upon whichprinting is to be performed;

FIG. 8 is a graphical representation qualitatively illustrating thepotential of a card, or substrate, with respect to time before printing,during printing, after printing and after removal of the card from theprinting location during a process which includes charging of the card,or substrate, both prior to and during printing;

FIG. 8-A is a graphical representation qualitatively illustrating thenet charge on a card, with respect to time, during a printing processperformed as in FIG. 8;

FIG. 9 is a graphical representation, similar to FIG. 8, but for aprinting process wherein the cards, or substrates. are charged onlyprior to printing;

FIG. 9-A is a graphical representation similar to FIG. 8-A but for aprocess as in FIG. 9 wherein charging occurs only prior to printing;

FIG. 10 is a graphical representation similar to FIGS. 8 and 9 exceptthat the card, or substrate, is simply interposed between a stencil andbacking electrode and only toner deposition during the printing processadds charge to the card;

FIG. 10-A is a graphical representation similar to FIGS. 8-A and 9-Aexcept that the printing process is as described with respect to FIG.10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates oneembodiment of apparatus for practicing the present invention. A coronasource, generally indicated by the numeral 10, is comprised of aconductive plate member 11 having a plurality of corona needles 12secured thereto. A conductive base electrode 14 is disposed in spacedparallel relation to the conductive plate member 11. Interposed betweenthe conductive plate member 11 and the conductive base electrode 14 area stencil screen 16, a substrate 18 which is to be printed upon and aconductive control grid 20.

The material with which printing isto be accomplished is illustrated asbeing toner 22 which is initially deposited upon the base electrode 14.The toner 22 may be fine granulated particles of conductive material,such as charcoal, or may be comprised of conductive carrier particles 23and nonconductive toner 24 (FIG. 2) which triboelectrically adheres tothe conductive carrier particles 23.

The stencil screen 16 is illustrated as being comprised of a conductive,fine wire-mesh screen 26 which has appropriate portions thereof blockedout by coating material 28 so as to provide openings 30 which permitpassage of toner 22, or 24, so that the toner can impinge upon substrate18 to print the desired pattern or image thereon. The substrate 18 iscomprised of relatively nonconductive material, e.g., as compared tometal, and is generally comprised of paper, cardboard, paperboard,plastics, or similar materials.

The conductive control grid 20 is illustrated as being a perforatedelectrode which can be generally described as a woven screen made up ofconductive wires such as might be used in conventional copper screen.

A power source 32 of direct current is provided and has connection means33 and 34 for connecting the power source 32 across the conductive platemember 11 and the conductive base electrode 14. Switch means 36 isprovided between the power source 32 and the conductive plate member 11,while switch means 38 is provided between the power source 32 and thebase electrode 14. Connection means 34 may also contain a secondarypower source 40 which can be connected in series with the power source32 by the switch means 38. A voltage divider 42 is connected across thepower source 32 and has an adjustable tap 44 which provides anelectrical connection to the conductive control grid 20.

OPERATION OF FIRST EMBODIMENT A substantially nonconductive card orsubstrate 18 is placed into the printing location in spaced relationwith the stencil screen 16 and the conductive control grid 20. Whenswitch means 36 is closed an electric field is established between thecorona source 10 and the conductive stencil screen 16. An

3 electricifield 'is-also established between the conductive controltgrid 20"and the corona source 10, as well as an electric field beingestablished between the conductive control grid'20 and; the conductivestencil screen 16. The electric field between the corona source and theperforated control grid produces. a corona dischargeatthe corona needles12. If the needles 12 are positive, they collect the negative ions whilethe perforatedcontrol grid 20 collects some positive ions. The electricfield-between thecontrol grid 20 and the conductive stencil screen 16causes positive ions to be deposited upon the substrate 18 and suchdeposition of positive ions occurs until the potential of thesubstrate18 reaches that of the control grid 20.. After the substrate 18-isuniformly charged, closingof the switch means 38 will cause toner 22 tobe charged inegatively andobe repelled by the baseelectrode 14. Some ofthetoner 22passes through; openings inthe stencil screen 16 and isdeposited upon the substrate l8. As the-buildupof'toner 22 onth'esubstrate-18 continues, the accumulation of negatively charged'tonertendsto repel deposition of additional tonerghowever, the originalpositive charge on the substrate l8-is-reduced becauseoftherneutralizing effect of the negatively charged toner and, since thepotentialof the substrate 18 is no longer the same as-thepotentialof'the control grid20, additional positive ions are depositedupon the substrate 18 from the corona source 10.

As a result, therepellingetfect of the negatively charged toner onthesubstrate 18 is diminished and additional toner 22 passes through theopening 30 and becomes deposited upon the substrate 18. A printingoperation as just described can be carriedout in something on the orderofone-twentieth vof-a second. After the toner 22is depositeduponthe-substrate l8, the substrate 18 is removed-from the printinglocation and the toner is fused-thereto by conventional processes-usingheator solventvapor. It-shouldbenoted that as the substrate '18-is,removed from the electric 'fieIdat the printing station, the toner 22will remain on the substrate l8because the accumulation of negativecharges resultingfrom deposition of charged toner 22 has beeneffectivelyneutralized by the addition of positive ionsunder control of-theconductive control grid or perforated electrode 20.

THE EMBODIMENT OFFIG. 2

FIG. 2isa diagrammatic illustration of a printing machine which utilizesthe present invention. A more complete disclosure of this printingmachine is set out in application Ser. No. 396,060, filed on Sept. 14,1964, and now US. Pat. No. 3,-306il93; A corona source, generallyindicated bythe numeral48, is similar-to thecorona source 10 but may becomprised of corona-wires50 insteadof corona needles 12. A conductivecontrol grid or perforated electrode 52 is interposed in spacedrelationbetween the corona source 48and a substrate 54: A conductivestencil screen 56, which isthe same as stencil screen 16, is-disposed-inspaced relation between the substrate 54 and a conductive base electrode581 The conductive particles 23 are preferably comprised of magneticmaterial and may-be uniformly-deposited upon the'base electrode58- by amagnetic drum'60 and-a magnetic roller 62. A conductive doctor blade64uniformlyspreads the carrier particles 23,- and nonconductive-toner24up'onsthe baseelectrode SS'and also providesfor an electricalconnectionfor establishing an elec- I trie field between thebaseelectrode 58'; thestencil screen 56; the control grid 52 and thecorona source 48. Suitableswitch means 66, '67, 68-and 69 are utilized for connecting thevarious'elements-tosuitable power sources-70, 71-, 72-and73. If desired,a.mechanical-connection, indicated by the broken line 74,- can couplethexsw-itch means 66, 67 and 68 for simul ta'neous-operation; Thejusbdescribedcircuitry is more particularly disclosedinthe-aforementioned application Ser. No. l

4 OPERATIONOF rne smaomtisnr on-Ftoz:

At the beginning of 'the pr-inting;operation, it is preferable that acorona discharge occur above the-substrate S t-(Corona wires 50 areprovided forproducing the' desired corona I discharge and anelectricfield is established betweenthe stencil screen '56 and controlgrid- 52'to, cause positive ions formed, by the-corona discharge to becarriedto thesubs'trate. 54:. This establishes a high intensityelectricjfield betweenthe substrate 54'and the stencil screen 5 6. Inaddition, thecharge deposited upon the substrate 54 tends toneutraliz'ethe field associated witha charged tonerimage-which is causedby accumulation of negatively charged-toner 24- upon the substrate 54.Thus, accumulation of toner 24.on. the substrate; 54:has littleinfluence on subsequently arriving toner 24s Durin'gthis printingoperation,v the carrierparticles 23 are charged by contact with the baseelectrode- 58]: and are repelledtherefrom' toward the stencil screen 56.The mesh size of the stencil 'screen'56' is such that carrier particles23 cannot pass therethrough but toneri24'passesthrough the openings inthe stencil screen 56 and become deposited upon the substrate 54.- Thecarrierparticles which are stopped by contact with the stencil'screen 56becomereversely charged and are repelled back toward base electrode 58sothat'th'e carrier particles oscillate between the base electrode. 58'andthe stencil screen 56 throughoutthe printing operation:

, After an image'of the desired optical'density'isdeposited A uponsubstrate 54; the sub'strate-54-is movedfrom thepriri -ti'ng zone toaconventionalfixing station where heater-solvent EMBODIMENT OE-FIG.

A potentially adverse characteristicrof the: apparatus and processes ofFIGS-Q Land =2=has to do with the timelrequired forchargeto-attain aproperly uniform.- distribution on=the surfaces of thesubstrates 18 andI54-which are to be printed; lf'the substrate to .be printed is'sufficiently' conductive so "that 1 the required time forthisuniform-.dis'tributionvto occurjisvery small, as compared to the-timesrequired to position thesubstrate,perforrnthe-print-ingoperatiomandtransfer the substrate out of'theprinting-zone, then there is noproblemi However, if a relatively longtimeis required; it'imposesa severe} limit on the printingrate- Ithasbeen found that under condi-., tions of low relative humidity, suchas up to 20 percent; printing rates greater than:l00impressions,penminute,could not be attained.

This time. problem can be. solved w -ma apparatus 1 and methodillustrated diagrammaticallyin"FIG. 3; 'Thismethod has beenpracticedby'simply usingtwo of the unitsipreviously. disclosed andconnecting the-units electrically a's shown FIG; 3. A firstcoronasource-'islprovided attheprechargin location and iselectricallyJconnected bywjre 8lto a-secondt corona source 82 which' islocated attheprintinglocation?A? first control 'grid-84 is situatedatthe -precharging location'ands is connected bywire. 8-5*to asecondcontrolgrid 8,6disposed at;

the printing location. A stencil screen-88 mayz'conveniently ex: tendbetween .the precharging location and the printin'g loca-w tion;ofcourse; the portion-ofjstencil screerii 88m thepri'ntin location isprovided-with openings 90 in the formofth desiredrimage. The coronasources 80 -and-82 are connected t the oppositeside of a power, source92 "from theiconductivei'i stencil screen '88. Control grids 84* and {86are connected I" across-the power source 92by a voltage. divider94*and-an ad justable tap 96'.-Suitable switch means-98*is provided-forclo's z ing the circuit.

OPERATION OF THE EMBODIMENT OF FIG. 3

A first substrate 100 is shown as being disposed at the printinglocation above that portion of the stencil 88 which contains an opening90 for defining the desired image to be printed. Of course, it isunderstood that a base electrode, such as 14 or 58, is located beneaththe stencil 88 in the same manner as previously described with respectto FIGS. 1 and 2. A second substrate 102 is located at a precharginglocation prior to entry into the printing location. The substrates 100and 102 are shown as being comprised of cardboard stock having spacedsheets of paper material 104 and 105 separated by an intermediate sheetof fluted paper material 106. The first substrate 100 has beenpreviously charged so that, for example, positive ions are uniformlydistributed on the side of substrate 100 adjacent to the stencil screen88. Upon closing the switch 98, toner will pass through theimage-defining opening 90, from the base electrode (not shown), andbecome deposited upon the lowermost surface of substrate 100.Simultaneously, ions will be emitted from the corona source 82 such thatcontrol grid 86 will allow additional ions to be deposited upon thesubstrate 100 in order to neutralize the charge buildup due to thenegatively charged toner being deposited upon the substrate. Alsosimultaneously, positive ions will be emitted from the corona source 80and, under the influence of control grid 84, will become deposited uponthe uppermost surface of substrate 102. After the printing of substrate100 has been completed, it is moved out of the printing location to afixing station (not shown) and the substrate 102 is moved into theprinting location. The time required for the first printing operationand the time required for movement of the substrates 100 and 102 providesufficient time for the ions on sheet 105 to become uniformlydistributed upon sheet 104. If the time lag is too small, the printedimage will have a striped effect along those portions of the flutes 106which engage the sheet 104. With a proper time sequence, it is possibleto improve the printing rate and obtain in excess of 100 impressions perminute. It has been found that charging the substrate at the precharginglocation is sufficient for some toners. With this arrangement, the onlyfunction the corona source 82 has is to maintain the substrate at thepotential of the perforated electrode 86 while toner is being deposited.For some toners, the total amount of charge borne by the toner issufficiently small that the potential of the substrate is notsignificantly altered. An example of such toner is that known as [PI9431; for other toners, such as Xerox 914, the corona source 82 isdesirable when extended solid areas are printed to a high opticaldensity. In this case, a considerable change in potential of thesubstrate can occur because of the relatively large amount of chargeassociated with the toner.

DESCRIPTION OF THE EMBODIMENT OF IG. 4

The method and apparatus illustrated in FIG. 3 has been demonstrated asbeing superior to that shown in FIGS. 1 and 2. However, the situationstill exists where charges are not being deposited directly onto thesurface that is to receive the toner. Thus, it is still required thatthe charge flow through a material that can have a low electricalconductivity. It is possible to acquire independence of conductivity bya direct application of ions onto the surface to be printed. Such amethod would work on material of any conductivity and at a rate limitedonly by the speed with which the substrates can be handled and printed.The time required for charge already deposited on the substrate to flowto, and become uniformly distributed on, the print receiving surface canbe eliminated as a rate-limiting factor. Such a method and apparatus hasbeen successfully tested and illustrated in FIG. 4.

Charging of the substrate is done by the same basic method as describedin the previously discussed approaches. A power source 110 and switchmeans 112 are connected across a back electrode 114 and a corona source116. A voltage divider 118 is connected by an adjustable tap 120 to aperforated electrode or control grid 122. A substrate 124 is disposed inspaced relation between the control grid 122 and the buck electrode 114.Closing the switch 112 will cause positive ions to be emitted fromcorona source 116 and be deposited upon the lowermost surface ofsubstrate 124 under the influence of control grid 122. The substrate 124and back electrode 114 combine to form a parallel-plate capacitor. Thecapacitance of this system is much greater than that of an isolatedsubstrate. Thus, for any given quantity of charge required on thesubstrate, the voltage to which it must be charged is less when usedwith a back electrode than it is without one. If the capacitance of thesubstrate-back electrode system is C, and the potential to which it ischarged is V, then these are related to the quantity of charge Q storedin the capacitor by the expression Q= CV. If the capacitance of theisolated substrate is Cc, and the potential to which it must be chargedto store the same quantity of charge as that given above is Vc, thenthese variables are related to those of the substrate-back electrodesystem by the expression CV= Cc Vc.

Now, since Cc is much smaller than C, Vc must be much greater than V. Asa practical matter, higher voltages aremore difficult to attain andmaintain. Attainment requires a power supply of higher voltage;maintenance may require a power supply of higher current capability.This latter point may not be immediately obvious. The upper limit on thepotential to which an object can be maintained in air depends to a largeextent on the rate of leakage of charge from the object due to breakdownof the air. Now this breakdown, and subsequent leakage of charge, isprimarily associated with discrete point of small radii on the objectsurface. Around these points the field intensity has the maximum valuethat it experiences near the object. These points are the deviationsfrom an ideally perfectly smooth surface that any practical object musthave. These flaws are so tiny that, from a practical point of view. theyare always isolated from nearby surfaces, such as back electrodes orcharging sources. Thus, the electrical field about a given point is afunction of only its radius and electrical potential. It is not afunction of the separation of such flaws from nearby objects. Theseflaws can produce discharges which may be continuous and may bemaintained sometimes by a power supply with an adequate currentcapability. But any attempt to raise the voltage beyond this range canresult in a catastrophic discharge to a nearby object. Thus, the highestcharging of an object can generally be accomplished by use of ahigh-capacitance system and a relatively low voltage. Voltages such as3-5 kilovolts are common OPERATION OF THE EMBODIMENT OF FIG. 4

Positive ions are deposited upon the lowermost surface of substrate 124upon closing the switch means 112. The substrate 124 can thenimmediately be transferred by known conveying means to a printinglocation above stencil while the previously printed substrate 132 isbeing removed. With the apparatus located as disclosed, improvedprinting rates can be attained, as compared with those of FIGS. 1, 2 and3.

EMBODIMENT OF FIG. 5

The printing machine, generally indicated by the numeral 140, isdescribed in detail in application Ser. No. 409,2l3, filed on Nov. 5,1964. Essentially, toner is charged by corona means 142 and expelledfrom tubes 144 so as to pass through openings 146 in a continuousbelt-type stencil screen 148 so as to be deposited on substrate 150 toproduce the desired image. Substrate 150 is carried by a conductiveconveyor means 152 which is electrically connected to a corona source154 by a power source 156 and switch means 158. The electrical contacts160 cause the conveyor means 162 to function as a back electrode in amanner similar to that of back electrode 114 in FIG. 4. Positive ionsare emitted from corona source 154 and become deposited upon thatsurface of substrate 150 upon which toner will be deposited by theprinting machine 140. It will be apparent that a voltage divider meansand perforated electrode or control grid may also be utilized in thesame manner as described with respect to FIG. 4.

DESCRIPTION OF THE EMBODIMENT OF FIG. 6

The previously discussed charging methods have all involvedelectrostatic deposition of ions generated in a corona discharge. Theelectric field between the control grid of the charge source and eitherthe back electrode of conductive stencil screen deposits chargegenerated in the corona discharge onto the substrate to be printed.However, the deposition need not be accomplished by electrostatic means.Charge can be conveyed from a corona discharge to an object to beprinted by a moving airstream. The method illustrated in FIG. 6 is quitesimilar to the first method described; the substrate is charged in theprinting location; it is fully charged prior to printing; charge shouldflow to and become uniform on the surface to be printed, prior toprinting; ions are deposited on the substrate during printing tocompensate for the toner-borne charge and maintain the card at aconstant potential; after printing, the card retains a net charging thatmaintains the toner image in location until the toner is permanentlyfixed to the substrate. This form of deposition can be substituted forelectrostatic deposition in each of the previously discussed chargingmethods.

One or more corona chambers, generally indicated by the nt metal 170 areconstructed of first conductive end members 172 and have at least oneopening 174 herein. The second conductive end members 176 are spacedfrom the end members 172 and an insulative sleeve 178 is connectedbetween each of the end members-l72 and 176 for defining a hollowchamber 180. A power source l82'and switch means 184 are connectedbetween the first and second conductive end members 172 and 176. Acorona discharge means, such as needles 186, are provided upon the endmembers 176 for providing a source of ions. It is to be understood thatend members 176 and needles 186 could be replaced by other types ofcorona discharge means such as fine wires. A flow of air is introducedinto hollow chambers 180 through hollow tube means 190 which isconnected to a source of pressurized air or other means for producing aflow of air such as fans, or the like.

OPERATION OF THE EMBODIMENT OF FIG. 6

A substrate 192 is located above a conductive stencil screen 194 whichhas openings 196 for producing the desired image. Upon closing switch184, ions are emitted by the corona needles 186 while air flowingthrough the hollow chamber 180 carries the ions outwardly through theopenings 174 in the end members 172 such that the ions become depositedupon the upper surface of substrate 192. This continues until thesubstrate becomes charged sufficiently to repel newly arriving ions.With a suitable time pause to allow the ions on substrate 192 to becomeuniformly distributed, toner is subsequently passed through openings 196in the stencil 194 and becomes deposited in the desired image upon thebottom surface of substrate 192. As illustrated, the corona means 186emits positive ions; with such an arrangement, negatively charged toneris used to produce the desired image. As the charged toner becomesattracted to and deposited upon the substrate 192, additional positiveions become deposited upon the substrate 192 and, after the printingoperation, the substrate 192 is moved to a fixing station and the tonerimage does not deteriorate prior to fixing.

DESCRIPTION OF THE EMBODIMENT OF FIG. 7

The embodiment of FIG. 7 is somewhat similar to that in FIG. 3 in that asubstrate 200 is charged at a precharging location, prior to being movedinto the printing location above stencil screen 202. Likewise, chargesare deposited upon that surface of substrate 200 upon which the tonerimage is to be deposited. Accordingly, faster printing rates areattained because there is no need to hesitate in order to permit thecharges to become uniformly distributed on the substrate. Also, lowconductivity of the substrate is no longer a factor.

A corona chamber, generally indicated by the numeral 204. is comprisedof conductive members 206 and 208 which are separated by an insulativesleeve 210 in order to provide a hollow chamber 212. Cdrona means 214are carried by end member 208 for emitting ions within the hollowchamber 212. Power source 216 and switch means 218 are connected acrossend members 206 and 208 such that, upon closing switch 218, a coronadischarge is caused within the hollow chamber 212. Air flowing throughtube 220 then carries the ions outwardly from the hollow chamber 212through an opening 224 in the end member 206. i

With this embodiment, it is preferable to provide a back electrode 230which may be grounded as at 232. The function of the back electrode 230is to reduce the repulsive field that charges already accumulated on thesubstrate 200 exhibit to subsequently arriving ions. In FIG. 6, theconductive stencil screen 194 satisfies this same function.

During the time that a toner image is being deposited upon a substrate232, the substrate 200 is being charged Subsequently, the substrate 232is moved out of the printing location to a fixing station whilesubstrate 200 is being moved into position adjacent the stencil screen202. The potential of the substrate in the printing .zone, relative tothe conductive stencil screen 202, is the same as that of the substraterelative to the back electrode 230 if the substrate is separated fromeach of these the same amount at the appropriate location.

EXPLANATION OF FIGS. 8-- 10 FIG. 8 is a graphical, qualitativerepresentation of the potential on the substrate, or card, during theintervals of I before printing, (2) during printing, (3) after printingand (4) after removal of the substrate,or card, from the printinglocation. The potential in FIG. 8 occurs during that printing methodwhere the card, or substrate, is charged from a corona source both priorto and during printing. It will be seen that once a substrate is chargedto a given potential (illustrated as being positive) the potential doesnot vary because during the actual printing, i.e., while toner is beingdeposited upon the substrate, negative charges associated with the tonerare balanced out by the addition of positive charges from the.

corona discharge. After printing, and after removal of the substratefrom the printing location, the potential remains the same because ofthe balance maintained between the positive charges from the coronadischarge and the negative charges associated with the toner.

FIG. 8-A graphically illustrates the net charge on the card, orsubstrate, for the same printing process as in FIG. 8. Before printing,the positive charges from the corona discharge produce a net chargewhich remains constant throughout the actual printing, after theprinting, and after removal of the substrate from the printing location.

FIGS. 9 and 9-A graphically illustrate the potential and net charge on asubstrate during the same time intervals as in FIGS. 8 and 8-A exceptthat the substrate, or card, are charged only prior to printing andcharge is not added during the printing except by the charged toner. Itwill be apparent that the potential and the net charge becomes lesspositive during the printing but then remains constant upon completionof the printing and after removal of the substrate from the printinglocation. The charge associated with IPI 943l toner is less negativethan that associated with Xerox 9l4 toner, but the potential and netcharge are both sensitive to toner characteristics.

FIGS. 10 and I0-A graphically illustrate that method in which the card,or substrate, is simply interposed between a stencil screen and abacking electrode without the addition of any charge being applied tothe substrate from a corona discharge. The substrate is held at apotential determined by the electrostatic field associated with theprinting machines.

As charged toner is applied during the printing, the potential.

of the substrate and the net charge on the substrate decrease. Thepotential and net charge remain constant after printing but thepotential immediately decreases upon removal of the substrate from theassociated electric fields while the net charge remains constant at itslowermost values.

It is readily apparent that the substrate, desirably, should be fullycharged prior to printing. In addition, during printing, additionalcharge is to be deposited on the substrate. Part of this charge is thatborne by the toner while the rest is that delivered by the corona tomaintain the card at the same potential as the perforated electrode orcontrol grid in spite of the toner depositions. If there is no leakageof charge from the card, the net charge addition during printing is nearzero; after printing, the substrate is removed from the printinglocation and, without leakage, the accumulated charge should remainconstant. This charge aids in maintaining the toner image on thesubstrate until permanent adhesion is produced by a fusing operation.After fusing, this charge can be neutralized by known means if desired.The electrical potential of the substrate before, during and afterprinting should be the same as that of the perforated electrode ifleakage is avoided.

While preferred forms and arrangement of parts have been shown inillustrating the invention, and preferred methods have been described,it is to be clearly understood that various changes in details andarrangement of parts and method steps may be made without departing fromthe spirit and scope of the invention as defined in the appended claimedsubject matter.

We claim:

1. A method of electrostatic printing comprising the steps of providinga substantially electrically nonconductive substrate to be printed upon,a first step of treating an area of said substrate with electricalcharges having a first polarity for providing said substrate with anelectrical potential, said area being greater than the area to beprinted, applying electrically charged toner having a different polarityto less than the treated area of said substrate, maintaining saidsubstrate at substantially said electrical potential by a second step oftreating said substrate for providing additional electrical charges tocompensate for the charge of the applied toner, and fusing said toner tosaid substrate.

2. A method as defined in claim 1, wherein said first step is comprisedof treating said substrate with ions on the side thereof which isopposite to the side upon which toner is to be applied.

3. A method as defined in claim 1 wherein said first step is comprisedof treating said substrate with ions on the side thereof upon whichtoner is to be applied.

4. A method as defined in claim 1, wherein said first step of treatingand said toner applying step are performed with the substrate at thesame location.

5. A method as defined in claim 1, wherein said first step of treatingis performed while said substrate is at a precharging location, and saidtoner applying step is performed while said substrate is at a printinglocation.

6. A method as defined in claim 1, wherein said first step of treatingis performed prior to the step of applying toner to said substrate.

7. A method as defined in claim 6 wherein said second step of treatingsaid substrate with electrical charges is performed simultaneously withthe step of applying toner to said substrate.

8. in an electrostatic printing apparatus for printing upon a substrate,electric field means for selectively depositing electrically chargedtoner on a substrate to form an image upon said substrate, said tonertending to migrate on said substrate in the absence of said electricfield prior to fixing said toner to said substrate, means for preventingsaid migration of said toner prior to fixing including separate meansfor depositing an electrical charge upon said substrate for neutralizingthe electric field associated with the charged toner which defines saidimage.

9. In an electrostatic printing apparatus as defined in claim 8, saidseparate means for depositing an electric charge being comprised ofcorona electrode means disposed in spaced relation to said substrate,and a control grid disposed in spaced relation from and between saidcorona electrode means and said substrate for controlling the flow ofions from said corona electrode means to said substrate.

10. in an electrostatic printing apparatus as defined in claim 8, saidmeans for selectively depositing charged toner being comprised of a baseelectrode, a conductive stencil screen disposed in spaced relationbetween said substrate and said base electrode, and means forestablishing an electric field between said stencil screen and said baseelectrode.

11. In an electrostatic printing apparatus as defined in claim 10, saidseparate means for depositing an electric charge being comprised ofcorona electrode means disposed in spaced relation to said substrate ona side thereof opposite from said stencil screen, and a control griddisposed in spaced relation from and between said corona electrode meansand said substrate to control the flow of ions from said coronaelectrode means to said substrate.

12. In an electrostatic printing apparatus as defined in claim 8, saidmeans for selectively depositing charged toner being comprised of astencil screen having a pattern of openings formed therein, means forpositioning said substrate adjacent to said stencil screen, and meansfor causing toner particles to pass through said openings and bedeposited upon said substrate in a pattern determined by said pattern ofopenings; said separate means being comprised of means for substantiallyuniformly coating" said substrate with ions prior to printing upon saidsubstrate.

13. in an electrostatic printing apparatus as defined in claim 12, saidmeans for substantially uniformly coating said substrate including acorona discharge means for coating said substrate with ions on a side ofsaid substrate upon which said charged toner is to be deposited.

14. In an electrostatic printing apparatus as defined in claim 8, saidseparate means being comprised of corona means for emitting ions,electrode means disposed in spaced relation from said corona means, andmeans for connecting a source of potential between said electrode meansand said corona means.

15. In an electrostatic printing apparatus as defined in claim 14, gridmeans located in spaced relation from and between said electrode meansand said corona means, and means for applying a potential to said gridmeans.

16. in an electrostatic printing apparatus as defined in claim 15,wherein said electrode means comprises a conductive stencil screenhaving openings therein defining an image to be printed upon saidsubstrate.

17. In an electrostatic printing apparatus as defined in claim 8,wherein said separate means comprises a corona chamber including a firstconductive end member having at least one opening therein, a secondconductive end member spaced from said first conductive end member, aninsulative sleeve connected between said first and second end membersfor defining a hollow chamber, means for connecting a source ofpotential between said first and second end members, corona dischargemeans on said second conductive end member for providing a source ofions, and means for providing a flow of air through said hollow chamberwhereby ions can be caused to flow through said opening in said firstconductive end member.

18. In an electrostatic printing apparatus as defined in claim 17,.anelectrode spaced from said corona chamber a distance just large enoughfor allowing a substrate to be interposed in slightly spaced relationtherebetween, and means for creating a potential difference between saidelectrode and said corona discharge means.

19. In an electrostatic printing apparatus as defined in claim 8, saidprinting apparatus including a printing station and a prechargingstation, said separate means being located at said precharging station,and being comprised of corona electrode means, a back electrode locatedat said precharging station and spaced from said corona electrode means,means for connecting a source of potential between said back electrodeand said corona electrode means, and means for moving a substratethrough said precharging station and into said printing station, saidcorona electrode means being located so that ions can be deposited onthe side of said substrate upon which toner is to be deposited at saidprinting station.

20. In an electrostatic printing apparatus as defined in claim 8, saidseparate means comprising a first conductive member and a secondconductive member disposed in spaced relation from each other, aninsulative sleeve connected between said first and second conductivemembers for defining a hollow chamber, means for connecting a source ofpotential between said first and second conductive members, said firstconductive member having at least one opening therein, said secondconductive member comprising a source of corona discharge, and means forproviding a flow of air through said hollow chamber whereby ions can becaused to flow through said opening in said first conductive member.

21-. In an electrostatic printing apparatus as defined in claim 8, saidseparate means comprising a first conductive member and a secondconductive member disposed in spaced relation from each other, means forconnecting a source of potential between said first and secondconductive members, needle means connected to one of said conductivemembers, control means disposed in spaced relation from and locatedbetween said first and second conductive members, and means for applyinga potential to said control means.

22. Apparatus as defined in claim 21, wherein said control meanscomprises a conductive mesh screen.

23. Apparatus as defined in claim 21, wherein said means for applying apotential to said control means comprises a voltage divider meansconnected across said means for connecting a source of potential betweensaid first and second conductive members.

24. Apparatus as defined in claim 23, wherein said control meanscomprises a conductive mesh screen, and adjustable tap means connectedbetween said conductive mesh screen and said voltage divider means.

25. In an electrostatic printing apparatus for printing upon asubstrate, electric field means for selectively depositing electricallycharged toner on a substrate to form an image upon said substrate, saidtoner tending to migrate on said substrate in the absence of saidelectric field prior to fixing said toner to. said substrate, means forpreventing said migration of said toner prior to fixing including meansfor neutralizing the electric field associated with the charged tonerwhich defines said image, said means for neutralizing including separatemeans for depositing an electrical charge upon said substrate prior toand simultaneously with the depositing of charged toner upon saidsubstrate.

2. A method as defined in claim 1, wherein said first step is comprisedof treating said substrate with ions on the side thereof which isopposite to the side upon which toner is to be applied.
 3. A method asdefined in claim 1 wherein said first step is comprised of treating saidsubstrate with ions on the side thereof upon which toner is to beapplied.
 4. A method as defined in claim 1, wherein said first step oftreating and said toner applying step are performed with the substrateat the same location.
 5. A method as defined in claim 1, wherein saidfirst step of treating is performed while said substrate is at aprecharging location, and said toner applying step is performed whilesaid substrate is at a printing location.
 6. A method as defined inclaim 1, wherein said first step of treating is performed prior to thestep of applying toner to said substrate.
 7. A method as defined inclaim 6 wherein said second step of treating said substrate withelectrical charges is performed simultaneously with the step of applyingtoner to said substrate.
 8. In an electrostatic printing apparatus forprinting upon a substrate, electric field means for selectivelydepositing electrically charged toner on a substrate to form an imageupon said substrate, said toner tending to migrate on said substrate inthe absence of said electric field prior to fixing said toner to saidsubstrate, means for preventing said migration of said toner prior tofixing including separate means for depositing an electrical charge uponsaid substrate for neutralizing the electric field associated with thecharged toner which defines said image.
 9. In an electrostatic printingapparatus as defined in claim 8, said separate means for depositing anelectric charge being comprised of corona electrode means disposed inspaced relation to said substrate, and a control grid disposed in spacedrelation from and between said corona electrode means and said substratefor controlling the flow of ions from said corona electrode means tosaid substrate.
 10. In an electrostatic printing apparatus as defined inclaim 8, said means for selectively depositing charged toner beingcomprised of a base electrode, a conductive stencil screen disposed inspaced relation between said substrate and said base electrode, andmeans for establishing an electric field between said stencil screen andsaid base electrode.
 11. In an electrostatic printing apparatus asdefined in claim 10, said separate means for depositing an electriccharge being comprised of corona electrode means disposed in spacedrelation to said substrate on a side thereof opposite from said stencilscreen, and a control grid disposed in spaced relation from and betweensaid corona electrode means and said substrate to control the flow ofions from said corona electrode means to said substrate.
 12. In anelectrostatic printing apparatus as defined in claim 8, said means forselectively depositing charged toner being comprised of a stencil screenhaving a pattern of openings formed therein, means for positioning saidsubstrate adjacent to said stencil screen, and means for causing tonerparticles to pass through said openings and be deposited upon saidsubstrate in a pattern determined by said pattern of openings; saidseparate means being comprised of means for substantially uniformlycoating said substrate with ions prior to printing upon said substrate.13. In an electrostatic printing apparatus as defined in claim 12, saidmeans for substantially uniformly coating said substrate including acorona discharge means for coating said substrate with ions on a side ofsaid substrate upon which said charged toner is to be deposited.
 14. Inan electrostatic printing apparatus as defined in claim 8, said separatemeans being comprised of corona means for emitting ions, electrode meansdisposed in spaced relation from said corona means, and means forconnecting a source of potential between said electrode means and saidcorona means.
 15. In an electrostatic printing apparatus as defined inclaim 14, grid means located in spaced relation from and between saidelectrode means and said corona means, and means for applying apotential to said grid means.
 16. In an electrostatic printing apparatusas defined in claim 15, wherein said electrode means comprises aconductive stencil screen having openings therein defining an image tobe printed upon said substrate.
 17. In an electrostatic printingapparatus as defined in claim 8, wherein said separate means comprises acorona chamber including a first conductive end member having at leastone opening therein, a second conductive end member spaced from saidfirst conductive end member, an insulative sleeve connected between saidfirst and second end members for defining a hollow chamber, means forconnecting a source of potential between said first and second endmembers, corona discharge means on said second conductive end member forproviding a source of ions, and means for providing a flow of airthrough said hollow chamber whereby ions can be caused to flow throughsaid opening in said first conductive end member.
 18. In anelectrostatic printing apparatus as defined in claim 17, an electrodespaced from said corona chamber a distance just large enough forallowing a substrate to be interposed in slightly spaced relationtherebetween, and means for creating a potential difference between saidelectrode and said corona discharge means.
 19. In an electrostaticprinting apparatus as defined in claim 8, said printing apparatusincluding a printing station and a precharging station, said separatemeans being located at said precharging station, and being comprised ofcorona electrode means, a back electrode located at said prechargingstation and spaced from said corona electrode means, means forconnecting a source of potential between said back electrode and saidcorona electrode means, and means for moving a substrate through saidprecharging station and into said printing station, said coronaelectrode means being located so that ions can be deposited on the sideof said substrate upon which toner is to be deposited at said printingstation.
 20. In an electrostatic printing apparatus as defined in claim8, said separate means comprising a first conductive member and a secondconductive member disposed in spaced relation from each other, aninsulative sleeve connected between said first and second conductivemembers for defining a hollow chamber, means for connecting a source ofpotential between said first and second conductive members, said firstconductive member having at least one opening therein, said secondconductive member comprising a source of corona discharge, and means forproviding a flow of air through said hollow chamber whereby ions can becaused to flow through said opening in said first conductive member. 21.In an electrostatic printing apparatus as defined in claim 8, saidseparate means comprising a first conductive member and a secondconductive member disposed in spaced relation from each other, means forconnecting a source of potential between said first and secondconductive members, needle means connected to one of said conductivemembers, control means disposed in spaced relation from and locatedbetween said first and second conductive members, and means for applyinga potential to said control means.
 22. Apparatus as defined in cLaim 21,wherein said control means comprises a conductive mesh screen. 23.Apparatus as defined in claim 21, wherein said means for applying apotential to said control means comprises a voltage divider meansconnected across said means for connecting a source of potential betweensaid first and second conductive members.
 24. Apparatus as defined inclaim 23, wherein said control means comprises a conductive mesh screen,and adjustable tap means connected between said conductive mesh screenand said voltage divider means.
 25. In an electrostatic printingapparatus for printing upon a substrate, electric field means forselectively depositing electrically charged toner on a substrate to forman image upon said substrate, said toner tending to migrate on saidsubstrate in the absence of said electric field prior to fixing saidtoner to said substrate, means for preventing said migration of saidtoner prior to fixing including means for neutralizing the electricfield associated with the charged toner which defines said image, saidmeans for neutralizing including separate means for depositing anelectrical charge upon said substrate prior to and simultaneously withthe depositing of charged toner upon said substrate.